Arterial cannulation is a commonly performed invasive procedure in the operation room, the emergency department, and in the intensive care unit. The indications include the need for continuous blood-pressure monitoring, frequent arterial blood-gas analysis, and repeated blood sampling for laboratory evaluation. This procedure can be challenging even in the best of hands. Traditionally, the artery is located by feeling the pulse of the patient. The pulse may, however be weak or absent in patients with hypotension, edema, obesity or local thrombosis due to previous arterial cannulation in the same location. Furthermore, the catheter may not be passed successfully into the artery, despite apparent good blood return on initial puncture, or hematoma and spasms of the artery may develop after failed attempts, thus making further attempts even more difficult. While ultrasound (US) is being used with increasing frequency for central venous access, fewer clinicians are familiar with US-guided arterial catheterization. The aim of this study is to investigate if ultrasound facilitates arterial cannulation in children ≤24 months compared with the palpation method and to investigate the potential extra costs/savings of introducing the method. This study hypothesizes that the ultrasound method will facilitate arterial cannulation in small children compared with the palpation method.